1. Field of the Invention
This invention relates to a multi-screw extruder, in particular a twin-screw extruder, comprising/counter-rotating screws inserted in a housing, whose flights form intermeshing crests and roots, and which have at least two axial sections of flights each differing in number and pitch.
2. Description of the Prior Art
Such extruders comprise two or more cylindrical or conical screws arranged one beside the other in an adapted housing, which screws are counter-rotatingly meshing with each other, so that geometrically sealed chambers are formed, which are moved in axial direction during the screw rotation, and a forced conveyance of the medium filling the chambers is achieved. In the plastics industry, such multi-screw extruders are used as plastics processing machines due to the possibility of a forced movement of the medium under the influence of pressure, so as to pressurize the plastic material, knead, melt, mix, homogenize or also degas the same and convey the same to the outlet, for which purpose screws with a varying number of flights and/or pitch may be used in dependence on the effects to be achieved. Accordingly, the screws form axial sections of flights differing in number and/or pitch, where up to now the axial sections whose transitions also involve changes in pitch and in the number of flights are completely separated from each other by annular grooves, which are formed by recesses in axially normal planes. These annular grooves should ensure that an unimpeded flow is maintained and the plastic material moving along the roots is not blocked due to a change in the number of flights effected by an occurring crest. Taking into account the usual manufacturing tolerances, the annular groove also facilitates the proper engagement of the crests and roots in the transition zone of the axial sections. However, the annular grooves lead to an undesired wear of the housing, as in the vicinity of the annular grooves the housing wall is subjected to wear in contrast to the remaining screw portions, and after a certain operating period protruding annular webs are left at the housing wall, which impede or even exclude above all the use of different types of screw in the existing housing. Moreover, the annular grooves are often disturbing for manufacturing the flights of adjacent axial sections by means of face milling cutters dimensioned larger than the width of the recess, and enforce the processing of the flight sections opening into the annular grooves by means of special face milling cutters having a smaller diameter. The same is true for manufacturing these flights by peripheral milling, peripheral grinding or whirling. To reduce the wear of the housing in the vicinity of the annular grooves, it has already been proposed to dispose the annular grooves in a plane inclined with respect to the screw axis, but such constructions could not gain acceptance due to the major manufacturing difficulties involved.
As is disclosed in DE 23 64 507 A, a change in the number of flights without annular grooves is known per se for single screws with a second root, so-called barrier screws, but these do not constitute two true flights, but only a second root, where the first root is sealed at the outlet end and the second root is sealed at the inlet end, so that the material conveyed must necessarily flow over the existing barrier from the first into the second root, in order to be molten under the occurring shear forces. There is no forced conveyance, but because of the single screw arrangement, which is necessary as a result of the rolling conditions, only a friction-related conveying effect.